1. Field of the Invention
The invention relates to an apparatus for adjusting the seat back angle, and more particularly to an apparatus that ensures a fixed inclination of the seat back and permits a resilient swing of the seat back. Moreover, the tightness thereof is adjustable.
2. Description of the Related Art
The conventional office chair is ergonomically designed to achieve an optimal sitting comfort. It would be better if the angle of the seat back is adjustable. As shown in FIG. 1, a conventional office chair 10 includes primarily a seat portion 11 that is supported by a support base 12. A seat back 13 is positioned at the rear side thereof. When the operator is tired, he can lean against the seat back for taking a rest. However, an L-shaped bar 14 is interposed between the seat back 13 and the seat portion 11. Therefore, the position of the seat back 13 stays unchanged, and an inclination for an optimal sitting comfort is not obtained when the operator leans against the seat back for taking a rest.
FIG. 2 illustrates another conventional office chair 20 whose structure is the same to the structure in FIG. 1 except the shape. As shown in FIGS. 3A and 3B, the office chair 30 has the function of adjusting the angle of the seat back. A retainer 34 is interposed between a seat portion 31 and the support portion 32. In other words, the retainer 34 is pivotally attached by pivoted bolts 321 to the top 323 of the support portion 32. Moreover, a spring 322 is positioned at the front end thereof for providing a resilient force when the retainer 34 is inclined backwards. The above-mentioned structure permits a backward inclination of the seat back 33. However, the seat back 33 and the seat portion 31 create an L-shaped backward inclination. It is not suitable for the operator who wants to lean his back backwards and does not want to raise the seat portion. The retainer 34 has to bear the whole weight when the seat portion is swiveled. Therefore, the entire structure becomes so heavy that the flexibility of the office chair is lost. Besides, the seat portion 31 that is inclined backward will easily lose the center of gravity. Therefore, the operator has to take care of inclination when sitting on the chair. Otherwise, the entire chair can tip over backwards.
As shown in FIG. 4, an office chair 40 disclosed in U.S. Pat. No. 6,149,236 includes an inclination-adjustment pneumatic spring 43 between a rear support 41 and a seat support 42 for providing a cushioning resilience for the rear support 41. The above-mentioned pneumatic adjustment structure will be practical when it is upright and used for adjusting the height of the seat support 42 on the shaft portion 44. However, the resilience and the flexibility to change with the to-and-fro swing of the operator's body are not optimal when it is applied to the rear support 41. Therefore, it requires further improvements.